The influence of neutron irradiation on the thermo­physical properties is related to the irradiation tem­perature and the number of defects generated in the crystal structure. At temperatures <1000 ° C, the electrical232-234 and thermal conductivity217 of tung­sten and tungsten alloys decrease with increasing irra­diation dose. However, at elevated temperatures such as those occurring in a fusion environment, the effect of neutron irradiation is strongly mitigated by anneal — ing.107 Complete recovery of defect-induced material degradation should occur at temperatures >1200 °C (see Figure 8).

In addition to defect generation, material degrada­tion is also related to the formation of transmutation products such as Re and Os, which in general exhibit poorer thermophysical properties. Transmutation — induced degradation increases with increasing tem­perature and irradiation dose, which makes it the most relevant process for the degradation of material properties for future fusion reactors such as DEMO.

Despite the potential for full recovery of the mate­rial defects mentioned above, void-induced swelling occurs. The results235,236 of tungsten and tungsten alloys show that the material’s volume increases with increasing irradiation temperature (<1050 °C).237 W-Re alloys exhibit significantly improved swelling behavior compared to pure W, with a local maxi­mum at ^750 °C. However, the swelling only amounts to <1.7% at 9.5 dpa.237 Therefore, a negligi­ble effect of swelling can be expected for the opera­tion of ITER. Experimental values do not exist at temperatures >1050 °C as expected for the operation of DEMO.